1. Field of the Invention
The invention relates to a welding method. More particularly, the invention relates to a welding method using a high-energy beam such as a laser beam, an electron beam, or an ion beam.
2. Description of Related Art
As one welding method to join a plurality of overlapped metal sheets together, a welding method by laser welding is widely used because it offers the advantages several advantages, for example, there is little processing stress, high-speed welding is possible, and few portions are affected by residual heat.
Japanese Patent Application Publication No. 2013-132686 (JP 2013-132686 A) by the inventors describes a laser welding method that forms a plurality of nuggets at a constant pitch p, on a virtual closed curve formed by a circle, for example. A strength equal to or greater than that of a circular or round weld having approximately the same diameter dimension as the closed curve is realized by appropriately specifying the pitch p and diameter d.
With the technology described in JP 2013-132686 A, when the diameter of the nuggets becomes relatively large, for example, deformation due to residual stress or thermal deformation during welding may occur. One known way to inhibit such deformation during welding is to first perform tack welding, and then perform final welding on or near the tack weld.
However, when final welding is performed on top of a tack weld, the deformation inhibiting effect by the tack weld is lost when the final welding is performed, and as a result, deformation may end up occurring after all. On the other hand, if final welding is performed near the tack weld and the tack weld is left even after final welding, the weak tack weld may peel after the final welding is performed. Although peeling of the tack weld does not pose a problem in terms of strength, an abnormal noise may be produced when the tack weld peels.
With a welding method described in Japanese Patent Application Publication No. 2009-241116 (JP 2009-241116 A), a continuous weld is formed by first spot welding overlapped metal material along a predetermined joint line, and then laser welding along the spot weld. JP 2009-241116 A notes that spot welding obviates the need for a jig or temporary fixing for momentarily holding when performing laser welding. That is, a spot weld also serves as a tack weld to inhibit deformation of an object to be welded at the time of laser welding.
With the technology described in JP 2009-241116 A, the spot weld corresponding to the tack weld protrudes from the continuous weld by laser welding that corresponds to the final weld. Here, with a highly conductive aluminum alloy sheet or the like, for example, the tack weld is weaker than the final weld. Therefore, when the tack weld protrudes from the final weld as described in JP 2009-241116 A, the weak tack weld may peel for some reason after final welding is performed. Although peeling of the tack weld does not pose a problem in terms of strength, an abnormal noise may be produced when the tack weld peels.